I found this on a Web site:
The easiest way to break this habit is to teach your dog a release command. This is done by grasping the Frisbee with one hand and grabbing your dog?s jaw just behind the Frisbee and squeezing. It is very important to use a command such as "drop" or "out" every time you do this. When he lets go, praise him like he?s just done the best dog trick of all time.
I haven't tried this, though. My high-energy Labrador Retriever, Lwaxana, is a Frisbee-retainer, and the thing that works best is to tempt her with another attractive toy (she loves squeaky rubber things,) and grab the Frisbee when her jaw goes slack while she contemplates the tantalizing choice!